1. Field of the Invention
The present invention relates to a backlight apparatus, a method for controlling the same, and an image display apparatus.
2. Description of the Related Art
Image display apparatuses, which use liquid crystal panels, are predominantly provided in recent years. The liquid crystal panel is not a self-emitting (self-luminous) device. Therefore, it is necessary to provide a backlight which uses a light source such as LED (light-emitting diode) or the like. In the case of the liquid crystal display apparatus, the luminance of the backlight is changed in some cases in order to regulate the luminance of a screen image (picture). PWM (pulse-width modulation) is exemplified as a method which is frequently used to adjust the luminance of the backlight. In this method, the backlight is turned ON and turned OFF in a certain cycle, and the ratio between the turned-on period and the turned-off period is changed to adjust the luminance of the backlight thereby. If the cycle of turning ON/OFF is long, then the light flashing is visually recognized by human eyes, and hence the flicker (flickering) is caused in some cases. Therefore, in the PWM control of the backlight, the backlight is generally subjected to the flashing at a high frequency of not less than 200 Hz.
In another viewpoint, the liquid crystal display apparatus is inferior in the response performance as compared with the self-emitting (self-luminous) device such as a CRT (cathode-ray tube) display, a plasma display and the like. Therefore, the moving image blur is sometimes conspicuous when a moving image is displayed. Various techniques have been suggested in order to reduce the moving image blur. A technique called “backlight scan” is present as one of such techniques.
The technique called “backlight scan” is the following technique. That is, the backlight is turned OFF in conformity with the scanning (scan) of the liquid crystal, and thus the moment, at which the liquid crystal is switched”, cannot be seen so that the moving image blur is reduced thereby. In order to carry out this technique, it is necessary that the flashing frequency of the backlight should be conformed to the scanning frequency (frame frequency) of the liquid crystal. For this reason, it is necessary that the flashing frequency of the backlight should be lowered to 60 Hz, for example, when the scanning frequency of the liquid crystal is 60 Hz. Therefore, when the backlight scan is performed, then the moving image blur can be reduced, while the flicker is conspicuous in some cases. Therefore, if the backlight scan is performed when a still image is displayed, then the flicker is merely conspicuous, and no merit is obtained. Therefore, when the still image is displayed, it is appropriate that the backlight is subjected to the flashing at a high frequency without performing the backlight scan.
For example, a situation is assumed, in which a moving image is displayed on only a part of a screen, or a situation is assumed, in which a moving image is displayed on an entire screen but the extent or magnitude (speed) of the motion differs depending on the area in the screen. In such a situation, a high quality screen image (picture) is rather obtained such that the backlight scan is performed for only the area of the moving image or the area in which the motion is provided in a large amount, and the flashing of the backlight is performed at an ordinary frequency (high frequency) without performing the backlight scan for the other areas (area of the still image or the area in which the motion is provided in a small amount).
Japanese Patent Application Laid-open No. 2005-99367 discloses a relevant technique, i.e., such a technique that a backlight, which corresponds to a frame block judged to provide a moving image, is flashed, while a backlight, which corresponds to a frame block judged to provide a still image, is always turned ON.
Japanese Patent Application Laid-open No. 2006-323300 discloses such a technique that the speed of a moving image is detected when a still image and the moving image are displayed in a mixed manner so that the duty ratio of PWM is shortened to mitigate the moving image blur for an area in which the motion is fast, while the duty ratio is prolonged to suppress the flicker (flickering) for an area in which the motion is slow.